A BCD process is a technique for monolithic integrated process, through which devices such as bipolar transistor (Bipolar), complementary metal-oxide field effect transistor (CMOS), and double-diffused metal-oxide field effect transistor (DMOS) can be fabricated on the same chip, abbreviated as BCD process. The BCD process is gaining more and more attention since the BCD process synthesizes respective advantages of the three types of devices above.
Products such as a system on chip (SOC) formed through the BCD process may apply to aspects such as automotive electronics, power management, etc. In such applications, there is often a need to integrate electrically-erasable programmable read-only memory (EEPROM) into the product. However, both traditional BCD process and EEPROM formation process are quite complex. Thus, how to embed EEPROM into the BCD process and how to optimize the process so as to simplify its process become a hot area for research in recent years.
The traditional EEPROM core structure (also know as bit cell) primarily includes two transistors, one of which being a high voltage (e.g., 15V) NMOS transistor as a selection transistor, and the other one being a storage transistor with a floating gate. The process for forming the high voltage NMOS transistor and the storage transistor is quite complex. Take an example of a product which includes a 3.3V low voltage CMOS transistor, a 15V high voltage CMOS transistor and a storage transistor (including three metal layers), then the number of photolithography layers shall be more than 23. The conventional BCD process is also very complex, including forming process for devices such as low CMOS transistors, middle and high voltage LDNMOS transistors, bipolar transistors, resistors, capacitors, etc. The number of basic photolithography layers for the BCD process (including two metal layers) is more than 21. Therefore, if the process for forming traditional EERPOM core structure is simply embedded into the BCD process, its process procedure would become too complex, making no sense to be applied in industry.